iotests: Add quorum case to test 110

Test 110 tests relative backing filenames for complex BDS trees.  Now
that the originally supposedly failing test passes, let us add a new
failing test: Quorum can never work automatically (without detecting
whether all child nodes have the same base directory, but that would be
rather inconsistent behavior).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20190201192935.18394-21-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>